The present invention relates to the replication of metal micro-scale parts, and more specifically, it relates to the production of sacrificial, electroplatable molds. Such electroplatable molds enable the efficient replication of metal micro-scale parts, which would otherwise require the iteration of synchrotron exposures and other costly or time-consuming processes.
A key challenge in LIGA (described below) is the replication of multiple sacrificial molds for the electroplating of metal parts/structures. Many applications require metal parts for mechanical, electrical or other reasons. The ability to replicate electroplatable plastic molds would eliminate the need for repetitive synchrotron exposures, a costly and time-consuming step, as well as the subsequent development step.
The production of micro-scale metal parts via LIGA (German acronym for lithography, electroplating and molding) is a multi-step process requiring mask production, synchrotron exposure of the polymethylmethacrylate (PMMA) substrate (typically PMMA bonded to a metallized silicon wafer or a solid metal plate), development of the PMMA, electroplating to fill the cavities left within the PMMA mold, lapping and final dissolution of the remaining PMMA. Such technology is described in U.S. Pat. No. 5,378,583. A constraining step in this process is the requirement for access to one of the very limited number of synchrotron facilities. If the electroplatable PMMA (or other plastic) molds could be replicated without the need for repetitive synchrotron exposures, this would provide an enormous savings in time and cost in the production of larger quantities of the desired metal parts.
Numerous approaches to this replication problem have been explored at Kernforschungszentrum Karlsruhe GmbH.
U.S. Pat. No. 4,541,977, entitled xe2x80x9cMethod For Producing Separating Nozzle Elements,xe2x80x9d is directed to a specific method for producing a complex multi-nozzle assembly including an array of micro-channels and nozzles held between top and bottom plates with corresponding inlets and outlets. This assembly is used for the separation of gaseous or vaporous mixtures. A process for the replication of the internal features is described which infuses a polymer into a master mold of the internal features so that the polymer contacts a metallic bottom plate. Upon removal of the master mold, the negative plastic mold remains adhered to the bottom plate due to the presence of dove-tailed inlets which lock onto the infused plastic. Subsequent electroplating of this structure provides a metal replicate of the original master features from which the plastic mold can then be removed.
U.S. Pat. No. 4,661,212, entitled xe2x80x9cMethod For Producing A Plurality Of Plate Shaped Microstructured Metal Bodies,xe2x80x9d provides a number of more general approaches for producing electroplatable plastic molds that rely on the use of metal or carbon filled PMMA formulations. Different methods are used depending on whether the features to be electroplated are contiguous or non-contiguous. Non-contiguous features require the casting of an unfilled non-conductive PMMA resin into the features of a master mold followed by a second overlay casting with a filled conductive PMMA. Upon removal of the plastic mold from the master mold, the filled PMMA overlay provides a conductive and electroplatable base to which the unfilled PMMA features are bonded. One variation on this approach, also described in the ""212 patent, involves the prefabrication of a two-layer PMMA substrate in which one layer is unfilled and insulating and the second, bottom layer contains a conductive filler and is therefore conductive. This two-layer substrate is embossed with a master mold such that the features of the master mold penetrate through the insulating unfilled layer into the conductive filled layer. Such two-layer substrates are also used in other patents referenced below. The ""212 patent describes another process suitable only for contiguous features in which the master mold is first dip-coated to apply a thin mold release layer to the feature tops and then similarly dip-coated in a conductive, filled PMMA formulation such that the feature tops only are coated. The wells between the features on this mold are then filled and covered with an unfilled, non-conductive PMMA material. Upon removal from the master mold, the contiguous conducting path of the filled PMMA layer allows electroplating of the desired metal replicate.
U.S. Pat. No. 4,981,558 titled xe2x80x9cProcess For The Reproduction Of A Microstructured, Plate-Shaped Body,xe2x80x9d discloses a process similar to that described in U.S. Pat. No. 4,661,212 with the addition of ultrasound to enhance penetration of the metal master mold through the insulating top layer and into the conducting PMMA bottom layer of a pre-formed two-layer PMMA substrate. Use of ultrasound permits the elimination of the heating and cooling steps normally involved in such embossing procedures.
U.S. Pat. No. 5,055,163, is titled xe2x80x9cProcess For Producing A Two-Dimensionally Extending Metallic Microstructure Body With A Multitude Of Minute Openings And A Tool Suitable For This Invention,xe2x80x9d describes a similar process to that described in U.S. Pat. No. 4,661,212. In the process described in the ""163 patent, a master mold with multiple tapered projections is embossed into a two-layer substrate in which the conducting lower layer might be a filled PMMA, another filled polymer or a metal having a low melting point. The tapered feature tips facilitate penetration of the master features through the top layer and into the conducting layer of the substrate. The use of cylindrical master tools in a continuous process and the use of ultrasound are also described.
U.S. Pat. No. 5,073,237 titled xe2x80x9cMethod Of Making Molds For Electrodeposition Forming Of Microstructured Bodies,xe2x80x9d discloses a method that overcomes some of the difficulties associated with the preceding processes by using a two-layer substrate that consists of a sputtered or vapor deposited film of metal or carbon on an insulating polymer base such as PMMA. During the standard embossing process, the metal film along the walls of the embossed features is stretched and disrupted to form a discontinuous and therefore non-conductive array of isolated spangles of the deposited film. The film in the bottom of the embossed features is not disrupted in this manner and provides a conductive contact for subsequent electroplating of the features. The features in this case must be contiguous, however.
It is important in the electroplating of micro-features with high aspect ratios that the walls of the electroplating mold be non-conductive. If the feature walls as well as the feature bases are conductive, the electroplating process will tend to close off the feature cavity before it has been completely plated up from the bottom. Such difficulties preclude the simple deposition of a metallic conducting film on the surface of a sacrificial plastic mold prior to electroplating or the use of conductive plastics in a standard embossing or injection molding process to form sacrificial molds. In the case of features having low aspect ratios, either of the above options is readily applicable.
U.S. Pat. No. 5,162,078, titled xe2x80x9cMethod Of Producing Microstructured Metallic Bodies,xe2x80x9d is directed to the removal by reactive ion etching of residual polymeric films in the bottom of plastic mold cavities, which would prevent electroplating on the conductive base supporting those features. Such residues are a potential problem when embossing through the two-layer substrates described in many of the above patents. The reactive ion etch is directed perpendicularly to the surface of the base plate to avoid degradation of the plastic features.
U.S. Pat. No. 5,676,983 titled xe2x80x9cTool For Making A Microstructured Plastic Mold From Which Structures Can Be Formed Galvanically,xe2x80x9d and U.S. Pat. No. 5,795,519 titled xe2x80x9cProcess Of Making A Microstructured Plastic Mold,xe2x80x9d again describe a two-layer substrate but provide an embossing master mold in which the walls of features formed in the master mold are smooth while the top surfaces of the features possess rough surfaces having points and ridges adapted to penetrate into the electrically insulating layer. This enhanced penetration allows the embossing tool to more efficiently expose the electrically insulating layer at the bottom of the embossed cavities.
None of the above processes provide a simple and versatile method of replicating either contiguous or non-contiguous features in a sacrificial plastic mold. Many require the pre-fabrication of specific plastic substrates, which contain a conducting layer adhered to a non-conducting layer with precise height requirements. Various techniques also have been used to ensure penetration of the tooled embossing features through the non-conducting layer into the conducting layer. Some of these techniques require the fabrication of special embossing tools with sharpened or roughened features. In addition, some of the techniques are applicable only to contiguous features, a major limitation.
There remains a need in the micro-fabrication art for a method of replicating metal structures that is simple, versatile, useful in replication non-contiguous features as well as contiguous features, and requires neither the pre-fabrication of specific plastic substrates nor special embossing tools as alluded to above. The present invention provides such a method, and, as such, represents a significant advance in the art of microfabrication.
Accordingly, it is a primary object of the present invention to address the above-described need in the art by providing a sacrificial plastic mold having an electroplatable backing useful in a method for replicating metal structures and parts.
It is another object of the present invention to provide a sacrificial plastic mold prepared using plastic forming technologies without need for repetitive lithographic exposures.
It is still another object of the present invention to provide a sacrificial plastic mold with more than one level of features, wherein the features in the different levels may be the same or different.
It is a further object of the present invention to provide a sacrificial plastic mold in which either contiguous or non-contiguous features may be electroplated.
It is still a further object of the invention to provide a method for making a sacrificial plastic mold having an electroplatable backing.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
The invention provides a sacrificial plastic mold having an electroplatable backing, and a method for making a mold. The mold comprises the cast product of the infusion of a castable liquid formulation through a porous metal substrate (e.g., a sheet, screen, mesh or foam) and into the features of a micro-scale master mold supporting and/or contacting the porous metal substrate. Upon curing of the castable liquid formulation, i.e., polymerization and/or solvent removal, and removal of the master mold, the porous metal substrate becomes embedded within the surface of the cast mold and a cast structure with features determined by the master mold will project from the surface of the porous metal substrate. This cast structure, in turn, provides a sacrificial plating mold that can be used to replicate the features of the original master mold. In particular, the porous metal substrate to which the cast mold is mechanically bonded provides an electroplateable backing, i.e., a conducting support suitable for electroplating processes. After electroplating, the plated metal can be lapped and polished and the sacrificial cast mold can be dissolved to leave the replicated metal structure bonded to the metal substrate.
If a metal structure is desired, the cast mold can be overplated and the overplated metal then lapped and polished to provide a metal base containing the electroplated features. Machining and polishing processes can then remove the porous metal substrate. Alternatively, the electroplated parts can be detached from the metal substrate by the use of different metals for the substrate and electroplating process and subsequent selective dissolution of the substrate metal. In another variation, an appropriate metal coating or strike is deposited on the porous metal substrate, before or after the casting process used to form the sacrificial plastic mold. That metal coating or strike can then be selectively dissolved to release the plated metal parts.
Both hot embossing and injection molding processes can also be used to infuse thermoplastic materials through the porous metal substrate and into the contacting mold. Both methods are discussed in copending U.S. patent application Ser. No. 09/765,078, filed Jan. 17, 2001 now U.S. Pat. No. 6,422,528B1.
In another embodiment, the invention provides a method for manufacturing the above-described sacrificial plastic mold. The method involves infusing a castable liquid formulation through a porous metal substrate and into the features of a micro-featured tool and then cured via polymerization and/or solvent removal to form a cast mold containing an electroplatable metal substrate. Such casting processes may be carried out by hand or by using resin transfer molding (RTM) or reaction injection molding (RIM) equipment. The use of evacuated mold cavities in conjunction with the automated equipment is desirable to reduce entrapped air. It is also desirable to degas the castable liquid formulations either by vacuum if the components are not highly volatile or by simply holding the liquid under ambient conditions for a period prior to casting. The arrangement of the porous metal substrate, the master mold and the infused castable liquid formulation will vary according to the process used.